Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps and backlights. Applications of OLED devices include active-matrix image displays, passive-matrix image displays, and area-lighting devices such as, for example, selective desktop lighting. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is at least partially light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward-biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers or polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
Full-color OLED devices may employ a variety of organic materials to emit different colors of light. In this arrangement, the OLED device is patterned with different sets of organic materials, each set of organic materials associated with a particular color of light emitted. Each pixel in an active-matrix full-color OLED device typically employs each set of organic materials, for example to form a red, green, and blue sub-pixel. The patterning is typically done by evaporating layers of organic materials through a mask. In an alternative arrangement, a single set of organic materials emitting broadband light may be deposited in continuous layers with arrays of differently colored filters employed to create a full-color OLED device. In addition, black-matrix materials may be employed between the color filters in non-emissive areas of the OLED device to absorb ambient light and thereby improve the contrast of the OLED device. Such color filter and black-matrix materials are known in the art and are employed, for example, in the LCD industry. The contrast improvement possible by providing a black-matrix material between light-emitting areas of the OLED device is limited by the relative size of the light-emitting areas and the areas between the light-emitting areas, i.e. the fill factor of the OLED device.
The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light emissive elements of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting OLED device. The present invention may be directed to either a top-emitting or bottom-emitting OLED device. However, in one embodiment, because of the limitations on a transparent electrode that are overcome in the present invention, a top-emitting OLED device is preferred.
In top-emitting OLED devices, light is emitted through an upper electrode or top electrode, typically but not necessarily the cathode, which has to be sufficiently light transmissive, while the lower electrode(s) or bottom electrode(s), typically but not necessarily the anode, can be made of relatively thick and electrically conductive metal compositions which can be optically opaque. Because light is emitted through an electrode, it is important that the electrode through which light is emitted be sufficiently light transmissive to avoid absorbing the emitted light. Typical prior-art materials proposed for such electrodes include indium tin oxide (ITO) and very thin layers of metal, for example silver or aluminum or metal alloys including silver or aluminum. However, the current carrying capacity of such electrodes is limited, thereby limiting the amount of power that can be supplied to the OLED materials, and hence the amount of light that can be emitted from the organic layers.
Referring to FIG. 11, a top-emitting OLED device as suggested by the prior art is illustrated having a substrate 10 (either reflective, transparent, or opaque). Over the substrate 10, a semiconducting layer is formed providing thin-film electronic components 30 for driving an OLED. An interlayer insulating and planarizing layer 32 is formed over the thin-film electronic components 30 and a patterned reflective electrode 12 defining OLED light-emissive elements is formed over the insulating layer 32. An inter-pixel insulating film 34 separates the elements of the patterned reflective electrode 12. One or more first layers 14 of organic material, one of which emits light, are formed over the patterned reflective electrode 12. A transparent second electrode 16 is formed over the one or more first layers 14 of organic material. A gap 19 separates the transparent second electrode 16 from an encapsulating cover 20. The encapsulating cover 20 is transparent and may be coated directly over the transparent electrode 16 so that no gap 19 exists. In some prior-art embodiments, the first electrode 12 may instead be at least partially transparent and/or light absorbing. Because suitable transparent conductors, for example ITO, have a limited conductivity, the current that may be passed through the organic layers 14 is limited and the uniformity of the light-emitting areas in an OLED device may be adversely affected by differences in current passed through various portions of the transparent conductor 16. As taught in issued U.S. Pat. No. 6,812,637 entitled “OLED Display with Auxiliary Electrode” Cok, an auxiliary electrode 70 may be provided between the light-emitting areas of the OLED to improve the conductivity of the transparent electrode and enhance the current distribution in the OLED. For example, a thick, patterned layer of aluminum or silver or other metals or metal alloys may be employed. However, the thick patterned layer of metal may not be transparent, requiring the auxiliary electrode 70 to be located between the light-emitting areas, limiting its conductivity and restricting the manufacturing tolerances of the OLED, thereby increasing costs. Likewise, a typical black matrix supplied over the OLED device is similarly limited to locations between the light-emitting areas, reducing the contrast of the OLED device.
A top-emitter OLED device as illustrated in FIG. 11 typically uses a glass substrate, a reflective conducting first electrode 12 comprising a metal, for example aluminum or silver, a stack of organic layers, and transparent conducting second electrode 16 employing, for example, indium-tin-oxide (ITO). Light generated from the device is emitted through the transparent electrode 16. In these typical devices, the index of the ITO layers, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 50% of the generated light is trapped by internal reflection in the ITO/organic EL element, 25% is trapped in the glass substrate, and only about 25% of the generated light is actually emitted from the device and performs useful functions.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. For example, Chou (International Publication Number WO 02/37580 A1) and Liu et al. (U.S. Patent Application Publication No. 2001/0026124 A1) teach the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has an optical index that matches these layers. Light emitted from the OLED device at higher than critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The efficiency of the OLED device is thereby improved but trapped light may propagate a considerable distance horizontally through the cover, substrate, or organic layers before being scattered out of the device, thereby reducing the sharpness of the device in pixellated applications such as displays.
There is a need, therefore, for an improved organic light-emitting diode device structure that improves the power distribution over the OLED device and within light-emissive areas of the OLED device, contrast, light output, and sharpness of an OLED device.